1. Field of the Invention
The present invention relates to a heat treatment apparatus and method for heating a semiconductor wafer, a glass substrate for a liquid crystal display, and the like (hereinafter referred to simply as a “substrate”) by photo-irradiation of the substrate.
2. Description of the Background Art
Conventionally, a lamp annealer employing halogen lamps has been commonly used in the step of activating ions in a semiconductor wafer after ion implantation. Such a lamp annealer carries out the activation of ions in a semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of the order of, for example, 1000 to 1100° C. In such a heat treatment apparatus, the energy of the light emitted from halogen lamps is used to raise the substrate temperature at a rate of about several hundred degrees per second.
In recent years, with the increasing integration of semiconductor devices, it has been desired that junctions be made shallower with decreasing gate length. It has, however, transpired that even if the above lamp annealer, which raises the temperature of a semiconductor wafer at a rate of about several hundred degrees per second, is used to carry out the activation of ions in a semiconductor wafer, a phenomenon still occurs where boron, phosphorous, or other ions implanted in the semiconductor wafer are deeply heat diffused. The occurrence of such a phenomenon gives rise to the apprehension that the junction may become deeper than the desired level, hindering good device formation.
With regard to this, U.S. Pat. Nos. 6,998,580 and 6,936,797 disclose techniques for raising only the surface temperature of an ion-impregnated semiconductor wafer within an extremely short period of time (several milliseconds or less) by irradiating the surface of the semiconductor wafer with flashes of light from xenon flash lamps (The term “flash lamp” as used hereinafter refers to a “xenon flash lamp.”) The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of the light emitted from xenon flash lamps is shorter than that of the light emitted from conventional halogen lamps, and it almost coincides with the fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with the flash light emitted from xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly with only a small amount of light transmitted through the semiconductor wafer. It has also transpired that the flash light emitted within an extremely short period of time such as several milliseconds or less allows a selective temperature rise only near the surface of a semiconductor wafer. Such an extremely quick temperature rise with xenon flash lamps will allow only the ion activation to be implemented without deep diffusion of the ions.
Now, as a result of high-energy ion implantation prior to such flash heating, a number of defects are introduced into a silicon crystal of a semiconductor wafer. Such defects tend to be introduced to a somewhat greater depth below the ion-impregnated layer. For the implementation of flash heating, it is hence desirable that not only the ion activation but also the repair of such introduced defects be carried out together.
However, in extremely quick irradiation where the time of light emission from the flash lamps is only about one millisecond, the speed of temperature rise at the surface of the semiconductor wafer is higher than the speed of heat transmission to the inside of the semiconductor wafer due to the thermal conductivity of silicon. This enables a temperature rise in the ion-implanted layer, but not to a depth at which defects are introduced. Nevertheless, if extremely high-energy light is emitted from the flash lamps, it would be possible, even with extremely quick irradiation for about one millisecond, to raise the temperature at a depth where defects are introduced and thereby repair those defects. However, there arises a problem in that the surface temperature would rise considerably, causing damage to the semiconductor wafer.
There has also been a suggestion to extend the time of photo-irradiation by the flash lamps to about several milliseconds by controlling the coil constant of a power supply circuit supplying power to the flash lamps. Such extension of the irradiation time to about several milliseconds is considered effective in repairing defects introduced during ion implantation, because it allows a temperature rise not only at the surface of the semiconductor wafer but also to a somewhat greater depth inside the semiconductor wafer. However, there is a possibility that extending the time of photo-irradiation by the flash lamps may cause the generation of new crystal defects because of a continuous temperature rise at the surface of a semiconductor wafer.